Gordon W. Gifford

The Neurophysiology of Functionally Meaningful Categories: Macaque Ventrolateral Prefrontal Cortex Plays a Critical Role in Spontaneous Categorization of Species-Specific Vocalizations

Neurophysiological studies in nonhuman primates have demonstrated that the prefrontal cortex (PFC) plays a critical role in the acquisition of learned categories following training. What is presently unclear is whether this cortical area also plays a role in spontaneous recognition and discrimination of natural categories. Here, we explore this possibility by recording from neurons in the PFC while rhesus listen to species-specific vocalizations that vary in terms of their social function and acoustic morphology. We found that ventral prefrontal cortex (vPFC) activity, on average, did not differentiate between food calls that were associated with the same functional category, despite having different acoustic properties. In contrast, vPFC activity differentiated between food calls associated with different functional classes and specifically, information about the quality and motivational value of the food. These results suggest that the vPFC is involved in the categorization of socially meaningful signals, thereby both extending its previously conceived role in the acquisition of learned categories and showing the significance of using natural categorical distinctions in the study of neural mechanisms.

Selectivity for the Spatial and Nonspatial Attributes of Auditory Stimuli in the Ventrolateral Prefrontal Cortex

Spatial and nonspatial auditory processing is hypothesized to occur in parallel dorsal and ventral pathways, respectively. In this study, we tested the spatial and nonspatial sensitivity of auditory neurons in the ventrolateral prefrontal cortex (vPFC), a cortical area in the hypothetical nonspatial pathway. We found that vPFC neurons were modulated significantly by both the spatial and nonspatial attributes of an auditory stimulus. When comparing these responses with those in anterolateral belt region of the auditory cortex, which is hypothesized to be specialized for processing the nonspatial attributes of auditory stimuli, we found that the nonspatial sensitivity of vPFC neurons was poorer, whereas the spatial selectivity was better than anterolateral neurons. Also, the spatial and nonspatial sensitivity of vPFC neurons was comparable with that seen in the lateral intraparietal area, a cortical area that is a part of the dorsal pathway. These data suggest that substantial spatial and nonspatial processing occurs in both the dorsal and ventral pathways.

Spatial and non-spatial auditory processing in the lateral intraparietal area

We tested the responses of neurons in the lateral parietal area (area LIP) for their sensitivity to the spatial and non-spatial attributes of an auditory stimulus. We found that the firing rates of LIP neurons were modulated by both of these attributes. These data indicate that, while area LIP is involved in spatial processing, non-spatial processing is not restricted to independent channels.

Discrimination of Functionally Referential Calls by Laboratory-Housed Rhesus Macaques: Implications for Neuroethological Studies

Prior to examining the neural correlates of auditory cognition with ethologically relevant stimuli, it is first necessary to establish that laboratory-housed animals respond to these stimuli with species-typical responses. Here, we report the results of experiments on laboratory-housed rhesus monkeys using both species-typical vocalizations and band-pass noise. Paralleling the approach used in field studies of this species, we used a habituation-discrimination paradigm in which auditory stimuli were presented and a monkey’s orienting responses to the stimuli were quantified. In parallel with the results obtained in field studies, we found that laboratory-housed rhesus classified species-typical vocalizations according to their putative referent properties as opposed to similarities in their acoustic morphology. In control experiments, monkeys oriented to band-pass noise but did not categorize differences in the spectral composition of the noise stimuli. These findings support the hypothesis that laboratory-housed rhesus classify, in the absence of training, species-typical vocalizations in a manner comparable to rhesus monkeys living under more natural conditions. As such, species-typical vocalizations are an appropriate and necessary class of stimuli in experiments that explore the neural correlates of auditory cognition in rhesus monkeys from a neuroethological perspective.

Auditory processing in the posterior parietal cortex.

Goal-directed behavior can be characterized as a dynamic link between a sensory stimulus and a motor act. Neural correlates of many of the intermediate events of goal-directed behavior are found in the posterior parietal cortex. Although the parietal cortexs role in guiding visual behaviors has received considerable attention, relatively little is known about its role in mediating auditory behaviors. Here, the authors review recent studies that have focused on how neurons in the lateral intraparietal area (area LIP) differentially process auditory and visual stimuli. These studies suggest that area LIP contains a modality-dependent representation that is highly dependent on behavioral context.